1. Field of the Invention
The present invention relates to a driving force control system in a vehicle, including a driving force control means for limiting the driving force on a driven wheel to prevent excessive slip of the driven wheel.
2. Description of the Prior Art
Driving force control systems in a vehicle are known as traction control systems, and have already been proposed such as in Japanese Patent Application Laid-Open No. 214974/1993.
Methods for limiting the driving force on the driven wheel in a traction control system generally include a method for reducing the output from an engine and a method for braking the driven wheel using a brake device. The former method includes the control of a throttle valve, the control of ignition timing, the control of the amount of fuel injected or the like.
In a vehicle having a traction control system using tandem throttle valves, a first throttle valve is mechanically connected to and is opened and closed by an accelerator pedal and a second throttle valve is disposed upstream of the first throttle valve and is opened and closed by a control motor. The driving force of the driven wheel is limited by driving the control motor with an electronic control unit to control the opening and closing of the second throttle valve.
In the prior art traction control system, when the driven wheel slips at the start of the vehicle movement and such slipping is continued to a certain extent, the slip rate is rarely maintained constant due to variations in the road surface friction coefficient, a variation in load applied to the tire, a variation in power output from the engine and/or the like and for this reason, a fine vibration is generated. If this vibration is operatively associated with the movement of the vehicle suspension system, a self-induced vibration (suspension judder) of the suspension system may be generated and in some cases provides an uncomfortable feeling to a driver, resulting in reduced riding comfort.